The principal focus of this laboratory is to characterize and quantitate lipoprotein-apolipoprotein metabolism at the cellular and subcellular levels. The evaluation of cellular receptors in cultured human skin fibroblasts and hepatocytes as well as the characterization of lipoprotein receptor function in these tissues isolated from patients with inborn errors of lipoprotein and apolipoproteins metabolism provides an understanding of normal as well as deranged lipid transport in man. Development of new techniques permit the direct study of cellular process which may be coordinately controlled: cholesterol synthesis through HMG-CoA reductase, cholesteryl ester hydrolysis by both acid and neutral cholesteryl ester hydrolase, the number and affinity of lipoprotein receptors for low density lipoproteins, apolipoprotein E, and high density lipoproteins, and the synthesis of nascent apolipoproteins and lipoprotein particles. Evaluation of cellular cholesterol metabolism in homozygous familial hypercholesterolemia, Wolman Disease, cholesteryl ester storage disease, abetalipoproteinemia and Tangier disease has been undertaken in both cultured skin fibroblasts as well as with isolated hepatic tissue. We have determined that the human hepatocyte manifests at least three receptors for lipoproteins: the LDL receptor, the HDL receptor, and the apolipoprotein E receptor. Abetalipoproteinemia, homozygous familial hypercholesterolemia, Wolman disease and cholesteryl ester storage disease interrupt cellular cholesterol homeostasis at entirely different levels within the hepatocyte. The compensatory alterations which subsequently occur indicate that the pathways of de novo cholesterol biosynthesis, cholesteryl ester storage, and cholesterol incorporation into nascent lipoproteins are coordinately regulated. An understanding of human cellular cholesterol homeotasis complements the clinical studies aimed at normalizing the plasma total and low density lipoprotein cholesterol concentrations.